Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The quality of the photocathodes is critical for the stable operation of the electron sources of the accelerator-based light sources (for example X-ray FEL at DESY [1], THz at ELBE [2], etc). In this field, high quantum efficiency (QE) and long operation lifetime are the most important parameters for the ideal photocathode materials. Cs2Te photocathodes have been successfully applied in the e- guns at DESY and in the superconducting RF gun at HZDR. Recently GaN as a new photocathode candidate has been paid more and more attention because of its high QE and robustness. However, even under the extremly high vacuum environment, photocathodes lose QE slowly and change the QE distribution during the operation. In this contribution, we will present the surface study on the Cs2Te and GaN photocathodes, including the QE evolution during operation (see Fig. 1) and the XPS analysis of the 'used' photocathodes (see Fig. 2). Several possible reasons leading degradation of those photocathodes might be: 1. rest gases during operation react with the sensitive alkali-rich surface; 2. the low energy ions, ionized rest gas molecules by the electrons, reach the cathode surface and react with the surface.